Halacaridae Explained

Halacaridae is a family of meiobenthic mites found in marine, brackish, and freshwater habitats around the world. It includes more than 1100 described species belonging to 64 genera ^{[1] [2]} It is the largest marine radiation of arachnids.^[3]

Description and life cycle

Halacarids have four pairs of legs (as adults and nymphs; see below), of which the first two pairs point forwards and the last two pairs point backwards. This is a synapomorphy of the group. Another synapomorphy are four plates on the dorsal surface of the body, except for several genera with reduction of certain plates.^[4]

The life cycle of halacarids consists of egg, larva, 1-3 nymphal stages (protonymph->deutonymph->tritonymph) and adult. Additionally, between each of the free-living stages (i.e. except for the egg) is a quiescent pupal stage.^[5]

• Eggs are usually deposited by an adult female in a substrate, with the help of an ovipositor.
• Larvae have three pairs of legs, with each leg five-segmented, and lack a genital plate.
• Protonymphs have four pairs of legs (as do all following stages), of which the fourth pair is five-segmented, and they usually have a distinct genital plate.
• Deutonymphs and tritonymphs have each leg six-segmented.
• Adults are often similar to the last nymphal stage, but they have an ovipositor (if female) or spermatopositor (if male).

Like mites in general, halacarids have a pair of palps. The palps usually have four segments each, but they are three-segmented in Simognathus and just two-segmented in Acaromantis.

Halacarids of subfamily Copidognathinae have just a single nymphal stage. Additionally, the number of genital papillae is reduced to a single pair.

The subfamily Rhombognathinae, which is algivorous, can be recognised by the dark green or almost black pigment inside their digestive system. This pigment is partially digested chlorophyll from algae.

Ecology

Halacaridae occur in various habitats including sandy beaches, tidal sediment, interstitial spaces, hydrothermal vents, mangroves, salt marshes and on larger animals.^[6] They spend their entire lives on a substrate such as attached algae or sand.

Most species and genera are predators, though Rhombognathinae are instead algivores. In freshwater halacarids, some species are restricted to crayfish gill chambers, implying a parasitic lifestyle, while Lobohalacarus weberi is a scavenger that feeds on dead nematodes and oligochaetes but not on live ones.

Phylogeny

Recent analyses place Halacaridae as the sister group to Parasitengona.^{[7] [8]} Within the group, algivorous Rhombognathinae consists of two lineages (Rhombognathus+Isobactrus and Rhombognathides+Metarhombognathus), meaning the habit of algivory has evolved two independent times.

Further reading

• Book: Breene . R.G.. Dean . D. Allen. Edwards . G.B.. Hebert . Blain. Levi . Herbert W.. Manning . Gail. 2003. Common Names of Arachnids. American Tarantula Society. Fifth Edition. 1-929427-11-5.
• Book: Capinera . John L.. 2008. Encyclopedia of Entomology. Springer. 978-1402062421.
• Book: Jackman . John A.. 2002. A Field Guide to Spiders and Scorpions of Texas. Gulf Publishing.
• Book: Krantz . G.W.. Walter . D.E.. 2009. A Manual of Acarology. Texas Tech University Press. 3rd Edition. 9780896726208.

Notes and References

1. Durucan . Furkan . New record of the genus Scaptognathus (Acari: Halacaridae) from Antalya with a checklist of marine halacarid mites of Turkey . Turkish Journal of Zoology . 2018 . 42 . 4 . 499–507 . 10.3906/zoo-1803-6 . free .
2. Bartsch . Ilse . Checklist of marine and freshwater halacarid mite genera and species (Halacaridae: Acari) with notes on synonyms, habitats, distribution and descriptions of the taxa . Zootaxa . 2009 . 1998 . 1–170 . 10.11646/zootaxa.1998.1.1 .
3. Pepato . Almir R. . Vidigal . Teofânia H.D.A. . Klimov . Pavel B. . 2018 . Molecular phylogeny of marine mites (Acariformes: Halacaridae), the oldest radiation of extant secondarily marine animals . Molecular Phylogenetics and Evolution . en . 129 . 182–188 . 10.1016/j.ympev.2018.08.012. 30172010 . free .
4. Harvey . MS . 1989 . Pezidae, a new freshwater mite family from Australia (Acarina : Halacaroidea) . Invertebrate Systematics . 3 . 6 . 771 . 10.1071/it9890771 . 1445-5226.
5. Bartsch . Ilse . 2015-02-17 . The genital area of Halacaridae (Acari), life stages and development of morphological characters and implication on the classification . Zootaxa . 3919 . 2 . 201–259 . 10.11646/zootaxa.3919.2.1 . 25781126 . 1175-5334.
6. Bartsch . I. . 2003 . Mangrove halacarid fauna (Halacaridae, Acari) of the Dampier region, Western Australia, with description of five new species . Journal of Natural History . en . 37 . 15 . 1855–1877 . 10.1080/00222930110089184 . 85063373 . 0022-2933.
7. Pepato . A R . Klimov . P B . 2015 . Origin and higher-level diversification of acariform mites – evidence from nuclear ribosomal genes, extensive taxon sampling, and secondary structure alignment . BMC Evolutionary Biology . en . 15 . 1 . 178 . 10.1186/s12862-015-0458-2 . 1471-2148 . 4557820 . 26330076 . free .
8. Dabert . Miroslawa . Proctor . Heather . Dabert . Jacek . 2016 . Higher-level molecular phylogeny of the water mites (Acariformes: Prostigmata: Parasitengonina: Hydrachnidiae) . Molecular Phylogenetics and Evolution . en . 101 . 75–90 . 10.1016/j.ympev.2016.05.004. 27150348 . free .